ALLISON 250-C SERIES
ENGINES LOOSE “B” NUTS

Transport Canada Civil Aviation (TCCA) is revising this SDA as a result of the Canadian Transportation Safety Board investigation into a Bell 206L accident in 2006. As demonstrated by this and other accidents and incidents worldwide, loose B-nuts on a pneumatic or fuel delivery tubes on the Allison/Rolls-Royce 250 series engines continue to cause in-flight loss of engine power. These failures can result in substantial damage to the helicopter and death or serious injury to the occupants.

Engine manufacturers’ maintenance documents provide specific information on the proper assembly methods and torque values. These documents also prescribe the use of torque stripes, witness marks and lockwire so that a fitting or nut that has come loose will be readily noticeable.

To attract the maintainers’ attention and highlight the importance of proper maintenance procedures, the manufacturers use appropriately placed NOTES, WARNINGS and CAUTIONS in the text of the related documents. Rolls Royce has also produced Commercial Service letters (CSL) for each model affected, latest revision February 2007.

TCCA reminds AMEs of the requirements to adhere to all the latest manufacturers’ instructions even when performing what may seem a rudimentary task. It is also recommended all owners/operators insure tasks outlined in the appropriate CSL above are immediately incorporated in the relevant maintenance schedule.